The mechanisms of CD8+ cytotoxic T lymphocyte (CTL)-mediated tumor regression in vivo remain to be fully understood. If CTL do mediate tumor regression in vivo by direct cytotoxicity, this may occur via two major effector mechanisms involving the secretion of perforin/granzymes and/or engagement of Fas by FasL expressed by the activated CTL. Although the perforin pathway has been considered the dominant player, it is unclear whether Fas-mediated cytotoxicity is additionally required for optimal tumor rejection in vivo. Accordingly, we examined whether a Fas/FasL-dependent CTL effector mechanism was important for optimal tumor regression in vivo. To that end, we developed a syngeneic mouse model of CTL adoptive immunotherapy against experimental lung metastases in both minimal (i.e., day 3 tumor-bearing mice) and extensive (day 10 tumor-bearing mice) disease settings. The model consisted of the CMS4 sarcoma, a solid tumor of BALB/c (H-2d) origin, and CMS4-reactive CTL, previously produced in immune competent mice using an anti-CTLA4 mAb-based strategy. CTL adoptive transfer resulted in nearly the complete regression of 3-day established experimental lung metastases. More importantly, under conditions of extensive lung metastases, we showed for the first time that (a) CTL adoptive transfer can promote significant tumor regression, and (b) the mechanism(s) of tumor regression required execution of a FasL-dependent CTL effector pathway. We propose that under such physiologic conditions, multiple immune effector mechanisms, in addition to the perforin/granzyme pathway, are necessary to achieve maximal antitumor reactivity. From a positive standpoint, combination strategies that involve the use of pro-inflammatory cytokines, certain chemotherapeutic agents or radiation, for example, to further sensitize neoplastic cells to T cell attack via regulation of Fas expression and function, may have implications for improving vaccine and/or passive-based antitumor regimens. In fact, in collaborative studies, we have demonstrated in pre-clinical models that local irradiation of a subcutaneously established tumor combined with Ag-based active or passive immunotherapies enhanced the tumor rejection response significantly more so than either modality given alone. Moreover, the immune mechanism(s) of tumor rejection induced by radiation exposure occurred via Fas-based interactions.